Adsorptive separations using zeolitic structures as adsorbents are well known in the prior art for resolving a multitude of gas mixtures. Such separations are predicated upon the compositions of the gas mixtures and the components' selectivity for adsorption on adsorbents, such as zeolites.
The use of nitrogen in industrial gas applications has seen significant growth, particularly with the development of non-cryogenic gas mixture separations. A major field of nitrogen separation comprises the separation of nitrogen from air. The removal of nitrogen from air results in an enriched oxygen gas component which is less strongly adsorbed by appropriate zeolites which are selective for nitrogen adsorption. When oxygen is desired as product typically at elevated pressure, it is desirable to adsorb nitrogen from air to result in unadsorbed oxygen enriched product passing over a nitrogen selective adsorbent. The nitrogen is then removed during a stage of desorption, typically at lower pressure. This results in oxygen being recovered at the pressure of the feed air, while nitrogen is recovered at a pressure below the feed air pressure. As a result, for the production of oxygen without significant pressure loss in an adsorptive separation of air, it is desirable to utilize nitrogen selective adsorbents, such as the family of zeolites.
Although various zeolites are naturally occurring and various synthetic zeolites are known, some of which have appropriate selectivities for nitrogen over oxygen and other less strongly adsorbed substances such as hydrogen, argon, helium, and neon, the industry has attempted to enhance the performance of various zeolites to improve their selectivity and capacity for nitrogen over such less strongly adsorbed substances such as oxygen.
For instance, U.S. Pat. No. 3,140,931 claims the use of crystalline zeolitic molecular sieve material having apparent pore sizes of at least 4.6 Angstroms for separating oxygen-nitrogen mixtures at subambient temperatures. U.S. Pat. No. 3,140,933 claims the use of lithium X-zeolite to separate oxygen-nitrogen mixtures at feed pressures between 0.5 and 5 atm and at a temperature between about 30.degree. C. and -150.degree. C. U.S. Pat. No. 4,859,217 claims a process for selectively adsorbing nitrogen using X-zeolite having a framework Si/Al molar ratio not greater than 1.5 and having at least 88% of its AlO.sub.2 tetrahedral units associated with lithium cations. This invention is based on the discovery that nitrogen adsorption from lithium X-zeolite at very high levels of lithium exchange is not predictable from the trend of the data obtained for samples with less than 86 equivalent percent lithium exchange and the remainder principally sodium.
Despite the performance of very high lithium exchange levels of X-zeolite for air separation, lower exchange levels of lithium would be desirable because it is costly to manufacture highly lithium exchanged materials. Large amounts of expensive lithium salts are required to prepare the highly lithium exchanged forms from the as-synthesized sodium form because the ion exchange of lithium for sodium is thermodynamically unfavorable.
The present invention overcomes the drawbacks of synthesizing high lithium exchange while still providing good performance as will be set forth in greater detail below.